1. Field of the Invention
This invention relates to an ink jet recording apparatus, and more particularly to an ink jet recording apparatus of the continuous jet type wherein ink is jetted continuously from a nozzle of an ink jet recording head.
2. Description of the Prior Art
Various ink jet recording apparatus are conventionally known and practically used. One of such conventional ink jet recording apparatus is of the continuous jet type wherein ink is jetted continuously from an ink jet recording head. An exemplary one of such conventional continuous jet type ink jet recording head is shown in FIG. 19. As shown in FIG. 19, the continuous jet type ink jet recording head shown includes an ink bottle 91 in which ink is accommodated, an ink pump 92 for applying a pressure to ink from the ink bottle 91 and sending out the thus pressurized ink, an ink tube 93 for supplying ink from the ink pump 92 therethrough, a nozzle 94 having a circular orifice of a very small diameter, an ink electrode 95 for holding the potential of ink in the nozzle 94 at a ground level, a vibrating element 96 in the form of a piezoelectric vibrating element mounted on the nozzle 94, a vibrating element driving vibrator 97 for applying an exciting signal to the vibrating element 96, a controlling electrode 98 having a circular opening or a slit-like opening coaxial with the nozzle 94 for receiving a controlling signal to control charging of a jet of ink, a grounding electrode 99 disposed in front of the controlling electrode 98 and grounded itself, a knife edge 100 mounted on the grounding electrode 99, a deflecting high voltage dc power source (hereinafter referred to as deflecting power source) 101, and a deflecting electrode 102 connected to the deflecting power source 101 for cooperating with the grounding electrode 99 to produce therebetween an intense electric field perpendicular to an ink jet flying axis to deflect a charged ink drop to the grounding electrode 99 side. The thus deflected charged ink drop is propelled to a record medium 104 wrapped around a rotary drum 103.
In such conventional continuous jet type ink jet recording apparatus, ink pressurized by the ink pump 92 is introduced by way of the ink tube 93 into the nozzle 94, at which a jet of the ink is formed from the orifice thereof. The ink jet is disintegrated into a train of ink drops with a spontaneous disintegrating frequency which depends upon a diameter and a flow rate of the ink jet and physical properties of the ink. In this instance, if the exciting frequency of the vibrating element 96 mounted on the nozzle 94 is set to a value at or around the spontaneous disintegrating frequency, then disintegration will be synchronized with excitation of the vibrating element 96, and consequently, ink drops of a very uniform size are produced in accordance with the exciting frequency.
Ink drops disintegrated in this manner are charged upon separation from the ink jet, by electrostatic induction by way of an integrating circuit composed of an electric resistance Rj of the ink jet and an electrostatic capacitance between the ink jet and the controlling electrode 98. Thus, if the controlling signal is a rectangular wave having an amplitude .phi.c, then a potential of an ink drop immediately before disintegration is given by EQU .phi.j-.phi.c(1-exp(-t/CjRj))
If the uniform ink drops separated from the ink jet are charge modulated in accordance with a controlling signal (recording pulse signal) synchronized in phase with an exciting signal, then such charged ink drops will be deflected to the grounding electrode 99 side by an action of the deflecting electric field and cut by the knife edge 100 while only non-charged ink drops are allowed to advance straightforwardly and pass by the knife edge 100 so that they form dots of ink on the record medium 104 wrapped around the rotary drum 103.
Now, if the exciting frequency (disintegrating frequency) is set to f.sub.d and an ink jet is pulse width modulated by a frequency of f.sub.d /n, then a picture image of n gradations with a controlled dot diameter can be recorded at the frequency of f.sub.d /n.
In the conventional continuous jet type ink jet recording apparatus described above, the exciting signal to the vibrating element 96 and the controlling signal (recording pulse) to the controlling electrode 98 must be synchronized with each other maintaining a certain optical phase relationship. In particular, while an ink dot is produced in synchronism with an exciting signal, a timing at which an ink jet disintegrates into an ink drop is varied delicately during one period of an exciting signal by a variation of parameters such as a temperature, an ink pressure and physical properties of ink. If such timing of disintegration and the controlling signal (recording pulse) are displaced in phase from each other, then the electric resistance Rj of an ink jet presents a very high value immediately before disintegration, and consequently, an edge of the controlling signal (recording pulse) comes within a region (hereinafter referred to as forbidden region) where the resistance is very high. Accordingly, charging of an ink drop takes place but incompletely, and an incompletely charged ink drop is produced. If an incompletely charged ink drop is produced, then it is impossible to individually control ink drops accurately. As a result, a spot-like noise is produced mainly at a highlight portion of a recorded picture image.
A technique of merely synchronizing an exciting signal and a controlling signal (recording pulse) with each other is disclosed, for example, in Japanese Patent Laid-Open Application No. 62-225363, Japanese patent Laid-Open Application No. 63-264361 and so forth.
Meanwhile, a method of determining an optimum phase between an exciting signal and a controlling signal (recording pulse) is disclosed, for example, in U.S. Pat. No. 4,839,665, wherein an ink jet is charged either in accordance with a probe pulse having a smaller width than a period (1/f.sub.d) of an exciting signal or another probe pulse having a pair of pulses having an equal amplitude and an equal pulse width within one period of such exciting signal but having the opposite polarities to each other while changing the phase of the probe pulse, and a current which flows together with an ink jet (such current will be hereinafter referred to as jet current) is successively measured to find out an optimum phase from measured values of the jet current. However, such jet current is a very low current (10 to 100 nA) and a current source is exposed to various noises. With an actual machine, it is difficult to shield such current source from external noises. Particularly, noises (hums) from a commerical power supply of , for example, ac 100 V matter.
A method of measuring a jet current is also disclosed in U.S. Pat. No. 4,835,665 mentioned above wherein a current detecting resistor is interposed between an ink electrode and the ground to convert a jet current into a voltage. Another method wherein an ink electrode is connected to a virtual grounding point of an operational amplifier constituting a current to voltage converter is disclosed in No. PCT/US88/03311. The two methods are advantageous in that, where a continuous jet type ink jet recording apparatus includes a plurality of nozzles like a color ink jet printer, a jet current can be detected independently for each of the nozzles. However, in order to introduce all of jet currents to a current detector, entire ink supplying systems from ink bottles to nozzles including ink pumps must be kept in an electrically isolated condition. Further, each of such ink supplying systems includes a very long ink tube and so forth and accordingly makes a very harmful noise source. Accordingly, it is difficult to measure a jet current at a high S/N ratio.
A further method of detecting a jet current flowing between a grounding electrode and a deflecting electrode is disclosed in U.S. Pat. No. 4,839,665 mentioned hereinabove. The method is superior to the method which makes use of an ink electrode in that a jet current can be measured at a high S/N ratio with low noises. However, it has the following problems:
(1) while measurement is easier on the grounding electrode side to which no high voltage is applied, in such instance, the grounding electrode, which is soiled with waste liquid, must be kept in an isolated condition; and
(2) even in a continuous jet type ink jet recording apparatus such as a color ink jet printer which includes a plurality of nozzles, only one deflecting electrode and only one grounding electrode are provided, and in this instance, since waste liquid from the nozzles come to the single grounding electrode, a jet current cannot be measured independently for each of the nozzles.
Also a method is disclosed in U.S. Pat. No. 4,839,665 mentioned hereinabove wherein an electrically isolated conductive ink catcher is provided in front of a grounding electrode and a deflecting electrode, and a current detecting resistor is interposed between the conductive ink catcher and the ground to detect a jet current. While the method is better then the two methods described above, since a signal source has a high impedance of 10.sup.9 to 10.sup.10 .OMEGA., also the current detecting resistor must be high in resistance, which makes it easy to admit noises. Consequently, measurement of jet current at a high S/N ratio cannot be achieved. Thus, an alternative measuring method using an ac technique, that is, a method wherein a probe pulse is amplitude modulated and a jet current is detected by means of narrow-band amplifier, is disclosed in U.S. Pat. No. 4,839,665 mentioned above. This method, however, still has a problem that a circuit system is complicated and expensive and the stability is low because an amplitude modulated probe pulse is used.
As described above, an ink jet printer such as a color ink jet printer normally includes a plurality of nozzles. In particular, where the conventional continuous jet type ink jet recording apparatus described hereinabove with reference to FIG. 19 is constructed as such ink jet printer, it includes a plurality of such continuous jet type ink jet recording heads as described above. In this instance, the continuous jet type ink jet recording heads are provided independently of each other while the grounding electrode 99, knife edge 100, deflecting power source 101 and deflecting electrode 102 are provided commonly to the ink jet recording heads. In such an ink jet printer, the nozzles 94 of the ink jet recording heads are disposed in line either in an axial direction (hereinafter referred to as the drum axial direction) or in a circumferential direction (hereinafter referred to as the drum circumferential direction) of the rotary drum 103.
By the way, since the nozzles 94 are different in directions of axes of ink jets therefrom (nozzle axes) and in flying speeds of such ink jets, they must be adjustable in registration. However, where flying speeds of ink jets are different, even if a controlling signal is received simultaneously by the controlling electrodes 98, times required for ink jets to reach a surface of the rotary drum 104 are different from each other. Consequently, the ink jets will be flown to displaced positions.
Adjustment in alignment of such nozzles 94 where they are arranged in line in an axial direction of the rotary drum 103 includes, as adjustment in a drum axial direction, mechanical leftward and rightward adjustment (in the drum axial direction) of the nozzles 94 and time lag adjustment of the recording picture element data for the nozzles 94 (by a distance between the nozzles 94), and includes, as adjustment in a drum circumferential direction, time lag adjustment of recording picture element data for the nozzles 94.
Adjustment in registration in a drum circumferential direction is conventionally achieved by either of the following two registration adjusting mechanisms:
(1) According to such registration adjusting mechanism as disclosed, for example, in Kent Bladh, Report 1, Dept. Electr. Meas., Lund Inst. Tech., 1982, pp. 112-114 or in Japanese Patent Laid-Open Application No. 62-225363, delay circuits having delay times adjustable independently of each other are provided for nozzles for four different colors (C (cyan), M (magenta), Y (yellow) and BK (black)). Each of the delay circuits is composed of a serial-in/serial-out type shift register and an oscillator having a variable vibration frequency and having an output to be supplied as a shift clock signal to the shift register. A time required until picture image data are outputted from the shift register after having been inputted to the shift register, that is, a delay time, can be adjusted by varying an output frequency of the oscillator.
(2) According to the other registration adjusting mechanism disclosed in Japanese Patent Laid-Open Application No. 62-33647, Japanese Patent Laid-Open Application No. 62-68761 and so forth, buffer memories (line buffers) are provided into which picture image data can be written at different addresses variable independently for four colors (C, M, Y and BK). Four color data are written into the buffer memories at different addresses which are displaced from each other by distances corresponding to distances between them, and reading out (printing) of data from the buffer memories is performed simultaneously for the four colors to compensate for the displacements of the nozzles.
With the first registration adjusting mechanism described above, if it is intended to raise the resolution for registration adjustment to assure a wide range of adjustment, then the oscillator frequency of the oscillator must be raised and the number of bits must be increased. Accordingly, a shift register which is high in number of bits and can operate at a high speed (for example, a several hundreds to several kilobits shift register which operates by several megahertz) is required. However, such shift register is expensive and is not readily available. Accordingly, a plurality of shift registers which do not have a sufficiently large number of bits must be connected in series in use.
Further, since the resolution in time is a reciprocal number to an oscillation frequency of the oscillator, if such frequency varies, then the resolution in registration adjustment is varied. Accordingly, in case registration adjustment is performed with a higher oscillation frequency so that a resolution for registration adjustment necessary at a minimum frequency may be assured, the resolution in registration adjustment may be unnecessarily high.
On the other hand, with the second registration adjusting mechanism described above, four color (C, M, Y and BK) picture data are written into the buffer memories at different addresses, and they are read out, upon printing, in synchronism with a picture element recording clock. Accordingly, a resolution in registration adjustment is a reciprocal number to a frequency of a picture element recording clock signal and is very low on a recording face because it is provided by recording dots on the recording face. Accordingly, the second registration adjusting mechanism is very low in resolution in registration adjustment and accordingly is not suitable for a high resolution printer.